EDINBURGH (Reuters) - A mother who has been fighting for four decades to find out what happened to her dead baby boy has discovered that his coffin has no human remains in it.

Lydia Reid's son Gary was seven days old when he died at Edinburgh's Hospital for Sick Children in 1975, she told the BBC.

Reid was a leading campaigner to expose how Scottish hospitals unlawfully retained dead children's body parts for research following a public enquiry into practices at Alder Hey Hospital in Liverpool, England in the 1990s, the BBC said.

"Lydia Reid wants answers from people ... somebody coming forward to explain to her what has happened to her son's body," David Short, a lawyer acting on her behalf, told the BBC on Monday.

Reid has suspected for years that her son's organs were taken without permission, and thought the coffin was empty on the day of the funeral because it weighed so little. But she has had no proof of what happened to his remains until now, following a court order for an exhumation at Saughton Cemetery in Edinburgh.

"I wanted to be wrong. I wanted to be called a stupid old woman but the minute (the forensic scientist) lifted the shawl out of the ground I knew there was nothing in it. Nothing," she said.

"My heart hit my feet and I didn't know what to say."

Reid has appealed to the Scottish health authorities and the funeral company to help her find out what happened.

"They know what happened to my son, they know fine and well that they have that knowledge and they can give me peace.

"Even if he's been incinerated I want to know. Even if he's lying in a jar in a hospital somewhere I want to know. If it's possible to get my son back, I want my son back. And if it's not, then at least tell me, and let me have peace."

Reid, now 68, said that after her son died and she asked to see him again she was shown a child that was not hers.

"This baby was blond and big, my baby was tiny and dark-haired. This was not my son," she said.

"I objected but they said I was suffering from post-natal depression."

Jim Crombie, Deputy Chief Executive at NHS Lothian, said: "Our condolences are with the family of Gary Paton. This matter is now being looked into by the police and we are unable to comment further."